The solid earth–the term we use for a globe which is most definitely not entirely solid–is a dynamic system. There are three fundamental layers. Outermost is the crust, which is solid, comprised of both continents and ocean floor. It is broken into plates, some very large like the Pacific or Eurasian plates, some tiny like the remnant of the Juan de Fuca oceanic plate being subducted beneath the Pacific Northwest (source of the Cascadian range of volcanoes). Continental crust ranges from 20 to 30 miles thick. The denser oceanic crust is roughly 10 miles thick.
Beneath the crust is the mantle, a plastic, semisolid layer about 1800 miles thick, and comprising about 84% of the volume of the planet. The rock in the mantle is extremely hot, but only liquefied in select locations. Minerals can change in the mantle, as a function of the heat and pressure–this is called metamorphism–but the texture of mantle rock is of a ductile solid, one which deforms and moves slowly, in huge convection currents which support and drive the motion of the plates overhead.
Beneath the mantle is the core, with a radius of nearly 2200 miles, comprised of a liquid outer and solid inner portion. Nuclear decay of uranium and other radioactive heavy metals are the source of the heat within the planet which drives all tectonic activity, and the very slow convective motion of iron in the outer core creates the magnetic field which permeates and surrounds the planet.
Think of planet earth as a gigantic spherical centrifuge, with the densest (uranium, iron, lead) elements gathered at the center, less dense silicates such as olivine, garnet and pyroxene in the mantle, and even less dense silicate rocks such as feldspar and quartz floating on top as the crust, with water and the gaseous atmosphere overhead. It is posited that, to a first (i.e. rough) order of approximation, the area of continental crust has been constant throughout most of earth’s history, as the minerals composing it have remained at their proper elevation in the centrifuge system. This concept lies behind most reconstructions of past continents.
Tomorrow: crustal motion.
This post was previously published on Dailykos.com and is republished on Medium.
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